1. Field of the Invention
The present invention relates generally to an exhaust gas purifying catalyst for use in an internal combustion engine and, more particularly, to an exhaust gas purifying catalyst including a porous monolithic carrier and a catalyst coating layer formed on the surface of the monolithic carrier.
2. Description of the Prior Art
Some conventionally known exhaust gas purifying catalysts include a porous monolithic carrier made of cordierite or the like and a catalyst coating layer formed on a surface of the monolithic carrier. The catalyst coating layer contains catalytic materials for expediting oxidation of hydrocarbons (HC), carbon monoxide (CO), or the like and reduction of nitrogen oxide (NOx) contained in exhaust gases. Materials containing noble metal components or other metal components, for example platinum, rhodium, and the like (these materials are hereinafter referred to as catalytic noble metals), are generally employed as the catalytic materials. In such conventional exhaust gas purifying catalysts, the catalytic noble metals are substantially uniformly distributed. However, the temperature and the exhaust gas speed generally vary from location to location in the exhaust gas purifying catalysts, while the exhaust gas purifying ratio of the catalytic noble metals depends on the catalyst temperature and the gas speed. Accordingly, the conventional exhaust gas purifying catalysts, in which the catalytic noble metals are uniformly distributed, cannot sufficiently enhance the exhaust gas purifying ratio.
Japanese Laid-open Patent Application No. 61-46252 discloses an exhaust gas purifying device, in which the distribution of concentration of a catalytic noble metal partially varies according to the distribution of temperature or that of gas speed in an exhaust gas purifying catalyst.
FIG. 1 depicts the structure of such an exhaust gas purifying device, which comprises a substantially cylindrical housing 102 communicating with an exhaust gas passage 101 and an exhaust gas purifying catalyst 103 disposed in the housing 102. The exhaust gas purifying catalyst 103 includes a monolithic carrier and a catalyst coating layer containing catalytic noble metals and formed on the surface of the monolithic carrier. This catalyst 103 is divided into three sections 104, 105 and 106, the first section (104) of which is high in concentration of the catalytic noble metals and the second and third sections (105, 106) of which are low in concentration of the catalytic noble metals. The third section 106 is substantially constant in width and is formed upstream from any other sections 104 and 105 in a direction of travel of exhaust gases. The first section 104 adjoins the third section 106 and has an elongated portion extending along the longitudinal axis of the catalyst 103 so that the longitudinal cross section thereof may be in the form of a figure "T". Around the elongated portion of the first section 104 is formed the third section 105.
However, when the engine is cold and the temperature of exhaust gases is low, the exhaust gas purifying catalyst 103 in the device of FIG. 1 is low in catalyst activity, thus lowering the purification ratio of the exhaust gases. Furthermore, the catalyst temperature near the longitudinal axis is generally higher than that of the circumferential portion. In the conventional catalyst 103, however, since the concentration of the catalytic noble metals is high at the location near the longitudinal axis of the catalyst, the temperature of this location becomes particularly high, thereby causing sintering of the catalytic noble metals, particularly platinum. As a result, the exhaust gas purifying catalyst 103 is thermally deteriorated.